Yes in general they noise limits do stop new builds, but, the futher away you are fro a town centre, the less noise will be created by the new structure, for example, i can place the biggest airport (noise 25), suffiently far enough away to enable it be placed wehn noise is 23 (it's usually connected to a rail station so it's not actually as far away as it looks.
I suspect in your picture, the newest airport is sufficiently far enough away from that town centre for it to gain a noise reduction for itself; probably affecting itself as 2 instead of 5 noise.